The present invention relates generally to gas turbine engines, and, more specifically, to high pressure turbine nozzles therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases from which energy is extracted in several turbine stages disposed downstream from the combustor. The high pressure turbine (HPT) first receives the hottest combustion gases from the combustor for powering the compressor, with additional energy being extracted therefrom in a low pressure turbine (LPT) which typically powers a fan in an aircraft turbofan engine application.
The HPT includes a stationary turbine nozzle which first receives the combustion gases from the combustor and channels them to first stage turbine rotor blades supported from a corresponding rotor disk. The turbine nozzle is protected from the hot combustion gases by air cooling using the pressurized discharge air from the compressor. The nozzle includes a row of hollow stator vanes having internal cooling circuits discharging air through a multitude of various holes extending through the sidewalls of the vanes.
Nozzle cooling is inherently complex in view of the three dimensional aerodynamic profile of the vane airfoils and the varying heat load over the pressure and suction side surfaces thereof. It is desirable to match the cooling air requirements against the experienced heat loads for minimizing the amount of cooling air diverted from the combustion process while correspondingly minimizing differential temperatures experienced by each vane and the resulting thermally induced stress therein.
However, the vanes themselves are directly exposed to the hot combustion gases and are mounted in relatively cool outer and inner bands, typically in circumferential segments for minimizing thermal stress due to differential thermal expansion and contraction. In a typical cooling arrangement, a perforate impingement baffle or insert is mounted inside each vane for directing cooling air in discrete jets against the inner surface of the vane for enhanced cooling thereof. The impingement baffle must be accurately spaced from the inner surface of the vanes for controlling impingement cooling, with stand-off features formed on either the baffle or the vane inner surface depending upon the particular nozzle design.
In one conventional design, each vane includes a mid-chord partition or bridge extending between the two sidewalls for defining forward and aft radial cavities in which corresponding impingement baffles are disposed. The two cavities are separately provided with portions of compressor discharge air for separately cooling the forward and aft portions of each vane to better match the external heat load.
However, the bridge is operated relatively cold since it is mounted inside the vane and is itself directly cooled by the air channeled inside the vane. The cold bridge is thusly joined to the two hot sidewalls at local regions subject to substantial differential thermal expansion and contraction due to the relatively cold and hot operating temperatures thereof.
Nevertheless, cold bridge turbine nozzle designs are common in commercial practice. For example, a turbine nozzle having two impingement baffles separated by a cold bridge has enjoyed successful commercial service in the United States for many years. This exemplary nozzle includes axially or chordally extending side ribs disposed inside each cavity on the opposite sidewalls thereof to provide stand-off features for centrally locating each impingement baffle in its corresponding cavity.
The side ribs stiffen or add strength to the thin sidewalls of the vanes and support the impingement baffles with relatively small clearances therebetween. The air discharged through the baffle holes firstly impinges the inner surfaces of the sidewalls, and is then confined radially by the ribs for discharge through inclined film cooling holes extending through the vane sidewalls in the forward and aft cavities, as well as for discharge through a trailing edge cooling circuit.
The trailing edge cooling circuit in the commercially used nozzle includes axially extending partitions or additional bridges which extend from the aft end of the aft cavity to the vane trailing edge. The partitions are spaced apart along the span of the vane to define corresponding trailing edge discharge channels terminating at corresponding apertures at the trailing edge. Radially extending turbulators are found in the trailing edge channels for enhancing the cooling effectiveness of the spent impingement air discharged through the trailing edge cooling circuit.
In the commercial nozzle example introduced above, the side ribs inside both vane sidewalls extend to and are integrally formed with the cold bridge. In this way the side ribs wrap around the comer junctions of the sidewalls and the cold bridge in a smooth transition.
The wrap-around side ribs at the aft end of the forward cavity provide an effective stand-off feature to limit the aft travel of the forward impingement baffle and maintain a minimum gap with the cold bridge. Correspondingly, the side ribs in the aft cavity along the vane suction sidewall also wrap or blend toward the opposite pressure sidewall where the two sidewalls converge closely together just forward of the trailing edge. Various ones of the trailing edge partitions extend locally forward and form integral continuations of respective ones of the side ribs for providing a corresponding stand-off feature to limit the aft travel of the aft impingement baffle.
In this configuration, the axially elongate side ribs are effective for circumferentially centering the individual impingement baffles in their respective vane cavities, with the aft blending of each side rib providing a convenient aft stand-off feature. And, the extended trailing edge partitions provide complete cold bridges across the two sidewalls in the aft cavity to limit axial travel of the aft baffle.
Although this commercial design has enjoyed many years of successful commercial service in the United States with a correspondingly large number of thermal cycles of operation, thermally induced cracking is being experienced which correspondingly limits the useful life of the nozzles requiring their replacement in a typical maintenance outage. Such cracking is being experienced in the vane sidewalls at the cold bridge, and in particular on the convex sidewall of the vane.
Accordingly, it is desired to improve the design of the turbine nozzle for increasing the useful life thereof.
A turbine nozzle includes vanes joined to outer and inner bands. Each vane includes opposite sidewalls extending between leading and trailing edges, with an internal bridge extending between the sidewalls to effect forward and aft cavities. Side ribs extend along the sidewalls and project inwardly into the cavities. Each of the side ribs is truncated adjacent the bridge to uncouple the ribs therefrom and increase nozzle life.